Someone that believes that there is no god. Some are more tolerant than others some are more fanatical than others, just like in every other group in the world.
PS: To the Christians, we tend to comment on you the most because you tend to attack the most. I have yet to run into a Buddhist monk ranting about how I'm going to suffer for all eternity for not being Buddhist.
Someone less tolerant of others: Haha. You're religious?! Stupid f*cking loser.
Someone more tolerant of others: Oh, I see. You're religious. I'm atheist.